Typical examples of earlier electromagnetic track brakes of this kind are shown in U.S. Pat. No. 3,768,607 and DE Pat. No. 928 106. Also in U.S. Pat. No. 2,564,945 and DE Pat. No. 2 627 794 examples are shown.
Different disadvantages may be found in all prior track brakes. A serious drawback common for the devices taken as examples of the prior art is that the means provided for taking care of the braking reaction forces in the frame and the shoes, which are individually movable to a certain extent, are not satisfactory, especially not if the length of the track brake is substantial, say 1 meter or more, or if the rail is irregular or has small radius curves. Prior art track brakes may even break down under severe working conditions due to a less favourable force and torque distribution in the frame and to deflections, which in prior art designs may be calculated to very high values.
The space available in the vehicle underframe for the track brake is often extremely limited, which eliminates simple solutions to the problem. A common requirement is also that the design shall be as cheap as possible but still extremely reliable under severe working conditions (for example a very harsh environment).